How is modern structural concrete different from the earlier form of concrete?
Modern structural concrete differs from Roman concrete in two important details. First, its mix consistency is fluid and homogeneous, allowing it to be poured into forms rather than requiring hand layering together with the placement of aggregate, which in Roman practice often consisted of rubble. Second, integral reinforcing steel gives modern concrete assemblies great tensile strength, whereas Roman concrete could depend only upon the strength of the concrete bonding to resist tension.
